Agreed,it wasn't a good argument.I tought about another one which I hope makes more sense:“God is real because he produces real effects.” If correct this conclusion can be applied to justify the existence of any other thing including an entity that is contrary to the existence of God(let's call it No-God),something on the lines:The believe in No-God produces real effects in the world therefore he is real.But No-God and God can't be both real therefore this conclusion can't be used to justify the existence of God.(this is a mere tought and probaly has strong counter arguments)

Another thing is that in the conclusion presented(God is real because he produces real effects.) it's not God that is producing the effects is the believe in God.

"Ultimately, William James argument in favor of the belief in God can be boiled down as:
“God is real because he produces real effects.” (p. 517)"

By anology the believe in Santa Claus by a child might have a positive and real effect on that child life the same way the believe in God does to some people.However that doesn't make Santa Claus real.